The present invention relates to a disk drive read circuit, and more particularly to an echo cancellation system employed in the disk drive read circuit to improve the quality of a signal detected by a transducing read head from a data track of a disk.
In the current state of technology, large amounts of data may be stored on data tracks of a rotatable disk as encoded magnetic transitions representing logical binary 0's and 1's. These magnetic transitions are detected, or read, by a sensor, or transducing head, suspended over the surface of the disk as it rotates. The sensor provides an input signal based on the detected magnetic transitions on the disk to preamplifier circuitry located at a distance from the sensor, with an electrical interconnect being provided between the sensor and the preamplifier circuitry. FIG. 1 is a simplified illustration of the underside of a typical disk drive configuration, including support structure 12 carrying slider 14 including transducing head 16 over the surface of a rotating disk. Sensor 16 is electrically connected to preamplifier circuit 20 by interconnect 18.
One problem with disk drive systems such as the one shown in FIG. 1 is that the input signal provided by head 16 is reflected by preamplifier circuit 20 due to an impedance mismatch between interconnect 18 and preamplifier circuit 20, and then reflected again by head 16 due to an impedance mismatch between interconnect 18 and head 16. As a result, the composite signal received by preamplifier circuit 20 includes both the true input signal and a delayed, distorted version of the input signal, which degrades the accuracy of the disk drive system in detecting magnetic transitions from the rotating disk. FIG. 2 illustrates the signal generated by head 16 as it travels and is reflected between head 16 and preamplifier circuit 20 along interconnect 18. The original input signal 22 is generated by head 16, and after one propagation delay (T) is received as signal 23 at preamplifier circuit 20. Signal 23 is the desired, undistorted signal that accurately represents the data transitions encoded on the magnetic disk. However, because interconnect 18 is not impedance matched to preamplifier circuit 20, the incoming signal is partially reflected back along interconnect 18 to head 16. Reflected signal 24 is received by head 16 after another propagation delay (T), and is attenuated and distorted based on the reflection coefficient of the preamplifier/interconnect interface (K.sub.PA). Reflected signal 24 is again reflected due to the impedance mismatch between head 16 and interconnect 18, resulting in re-reflected signal 25 being received at preamplifier circuit 20 one more propagation delay (T) later. Re-reflected signal 25 is further attenuated and distorted according to the reflection coefficient of the head/interconnect interface (K.sub.MR). The reflection process continues until the attenuation of the signal reaches negligible levels, and composite signal 26 received by preamplifier circuit 20 therefore includes undesirable reflected components that degrade the circuit's ability to accurately detect data transitions encoded on the disk.
One potential solution to the reflection problem is to impedance match either the preamplifier/interconnect interface or the head/interconnect interface. However, impedance matching the preamplifier/interconnect interface inherently introduces additional noise into the system, and because of the low signal levels utilized in disk drive technology, such impedance matching would reduce the signal-to-noise ratio of the system to an unacceptable level. Impedance matching the head/interconnect interface is not practical because the impedance of the head is not a tightly controlled parameter in manufacturing. Therefore, there is a need in the art for a solution that eliminates undesired reflected signals at the disk drive preamplifier while maintaining an acceptable signal-to-noise ratio to accurately detect the data transition signals encoded on the disk.